Little is known concerning the biodegradation of toxic materials in biofilm treatment systems, particularly at the mechanistic level. It is of vital importance because many materials which pose a significant public health threat are continually discharged into our sewage systems without knowledge of their ultimate fate. Nearly all research on biodegradation of toxics is done using suspended growth systems, such as the activated sludge process, rather than in biofilms. This is unfortunate, since most innovative treatment processes being developed for treating toxic wastes are based on the use of biofilms. In addition, there is a dearth of knowledge on the fate of organo-metallics in any media, but particularly in biofilms. The overall goals of this research program are to study biodegradation of toxic organics and organo-metallics in microbial biofilms by evaluating transport mechanisms of contaminants into and out of the biofilm, spatially and quantitatively characterizing the microbial populations in the biofilm, and applying the results obtained to full- scale treatment systems. A variety of azo dyes will be used as the test toxic substances. The end result should be a much clearer understanding of the mechanics of biofilm treatment systems, allowing for more accurate design of processes for treating toxic wastes, and thus protecting and preserving human health and the environment. Specific aims of the research are to: (1) characterize the biofilms with respect to variations in biofilm density, porosity, diffusivity, diffusion layer characteristics and microbial activity in and without the presence of the toxicant; (2) characterize the diffusion boundary layer at the biofilm/liquid interface, which controls the amount of substrate, toxic materials and nutrients reaching the microorganisms in the biofilm; (3) evaluate transport mechanisms and biodegradation kinetics for the azo dyes in the biofilms using microelectrodes and a micro-sectioning technique;(4) determine the location and number of azo dye-degrading microorganisms in the biofilms using microelectrodes and molecular probes specific for the degradative bacteria; (5) evaluate the impact of seeding aerobic biofilm reactors with azo-degrading microorganisms; (6) evaluate transport mechanisms and biodegradation kinetics for toxic metal- complexed azo dyes; and (7) evaluate the scale-up of micro-scale results to full-scale.